In the prior art, several methods are employed for the liquefaction of natural gas feeds. Among these is a multicomponent refrigeration cycle in which natural gas feed at ambient temperature, normally of about 100.degree.F. is successively cooled and liquefied in a plurality of heat exchange stages, resulting in a liquefied natural gas product at a temperature of about -260.degree.F. The cooling medium is a multicomponent refrigerant.
In the prior art method, the multicomponent refrigerant, having given up its residual cold in a first heat exchange stage, enters a compressor in the vapor phase, at a pressure of about 1-5 atmospheres and a temperature essentially ambient. By ambient temperature is meant the average temperature of the surrounding environment and thus, as applied to process streams, it is the temperature which can be closely approached by contacting those streams with the air, water, etc. The refrigerant is pressurized and cooled by heat exchange with water or air to form a two-phase mixture at a temperature slightly above ambient. This two-phase mixture is cycled to a separation drum upstream of the first heat exchange stage. The two phases are separated in this drum and both vapor and liquid phases enter the first heat exchange stage along with the natural gas feed. All of the above three streams are cooled by the recycled multicomponent refrigerant stream. This recycled refrigerant stream enters the first heat exchange stage as a two-phase mixture countercurrent to the three streams, generally at a temperature 0.degree.F. or below. In the course of cooling the three streams, the recycled refrigeration stream is warmed, exiting as a gas as discussed above at a temperature about ambient. Typical of the prior art method is U.S. Pat. No. 3,593,535 to Gaumer et al.
The disadvantages of this prior art method lie first in the high power requirements of the refrigeration compression step. A second disadvantage is the high capital cost of heat exchange stages. All the heat exchange stages in the prior art method are constructed of high cost alloy materials. This is necessary in order to insure that all heat exchangers and heat exchange stages operate without danger of rupture in the low temperature environment that they are subjected to in the prior art process.